The present invention relates to a themoplastic polyester resin composition having excellent toughness and mechanical strength, and more particularly to a thermoplastic polyester resin composition capable of providing molded articles having an improved impact resistance and other excellent properties such as tensile strength, elongation and heat resistance.
Representative thermoplastic polyester resin is polyethylene terephthalate. Polyethylene terephthalate has excellent mechanical properties and excellent chemical properties and, therefore, it has been widely used as materials for fibers, films and moldings.
However, polyethylene terephthalate has the disadvantages that the rate of crystallization and impact resistance are relatively low. Since the crystallization rate is relatively low, the injection molding must be conducted at a mold temperature higher than 140.degree. C. Comparing with other polyester resins, e.g. polybutylene terephthalate for which the mold temperature in injection molding is about 40.degree. C. or higher, polyethylene terephthalate requires a higher mold temperature.
It is known to use various kinds of crystallizing accelerators in order to improve the above disadvantages. For instance, Japanese Patent Publication Kokoku No. 48-4097 and No. 48-4098 propose to incorporate metal salts of organic acids, and Japanese Patent Publication Kokoku No. 55-47059 proposes to incorporate a plasticizer and an ionomer. The use of these crystallizing accelerators is effective for lowering the mold temperature in injection molding to a practical level, but the mechanical strength and impact resistance are not sufficiently improved.
It is also proposed to introduce a soft segment to polyethylene terephthalate by copolymerization in order to raise the crystallization rate simultaneously with an improvement in impact resistance. For instance, Journal of Polymer Science, Vol. 8, 1 (1952) discloses a copolymerization with polyethylene glycol. Japanese Patent Publication Kokai No. 61-243854 discloses a copolymerization of polyethylene terephthalate with an addition polymerization product of an alkylene oxide to a polyalkylene glycol or a bisphenol compound. The introduction of a soft segment, particularly polyalkylene glycols, improves the impact resistance of molded articles, but the mechanical strength is lowered.
An improvement in the impact resistance of molded articles of thermoplastic resins has been made generally by blending thermoplastic resins with elastomers. To thermoplastic polyester resins, including polyethylene terephthalate, which are one of engineering plastics, the method of blending elastomers has been also applied. It is proposed to blend thermoplastic polyester resins with copolymers of an .alpha.-olefin and an .alpha.,.beta.-unsaturated carboxylic acid glycidyl ester in an attempt to improve the impact resistance of polyester moldings, as disclosed in Japanese Patent Publication Kokoku No. 58-47419 and Japanese Patent Publication Kokai Nos. 53-117049, 58-17148, 59-115352 and 61-207458. However, these proposals have the disadvantages that no satisfactory improvement in impact resistance is obtained for some polyester resins. No theory about what elastomers are effective for improving the impact resistance has been established. Also, there are cases where if high performance engineering plastics are blended with elastomers having a relatively low elasticity, the impact resistance is improved, but excellent other properties expected as engineering plastics are impaired. Accordingly, it is the present state that experiments must be repeated by combining the objective thermoplastic polyester with various kinds of elastomeric materials in order to look for the suitable elastomeric materials.
It is an object of the present invention to provide a thermoplastic polyester resin composition capable of providing molded articles having well-balanced excellent properties at a low mold temperature.
A further object of the invention is to provide a composition capable of providing molded articles of polyethylene terephthalate resins having an excellent balance between mechanical strength, elongation, impact resistance and heat resistance.
A still further object of the invention is to provide a composition capable of providing molded articles of thermoplastic polyester resins having an improved impact resistance without substantially impairing mechanical strength, heat resistance and chemical properties.
These and other objects of the present invention will become apparent from the description hereinafter.